Construction of a 2-K.W. Transformer.
Core.—Strips 2 1/2 inches wide are cut from soft sheet iron. One half of them should be 11 1/2 inches long and the other half 6 1/2 inches. Enough are cut to make two piles of each size 2 1/2 inches high. Both sides (the longest strips) of the core are built up with the ends overlapping as in Fig. 41.
The ends (the short pieces) are then slipped between the overlapping ends of the long strips and the whole core squared up. The completed core should have a cross section of 2 1/2 inches square and form a hollow rectangle 8 x 14 inches. The strips of iron must be dipped in P. & B. insulating varnish and dried before they are assembled.
Primary.—Four fiber heads 6 inches square, 1/2 inch thick and having a hole 2 1/2 inches square are made. One of these is placed on the core legs as shown by Fig. 48. Two or three layers of well varnished linen are wound over the core preparatory to winding the primary. Room must be left for the other head to be placed on the opposite end after the windings are all in place. The primary consists of 100 double turns of No. 12 B. S. gauge double cotton covered magnet wire. Fifty of the double turns are wound on each leg. The four terminals of the primary are led out through the fiber heads. Care should be taken to distinguish them from each other so that they may be identified when the transformer is completed. The primary is then wound with a strip of micanite or empire cloth 8 inches wide until it measures 4 inches square over all.
Secondary.—The secondary coils, which are eight in number, are wound on a form 4 inches square and 1 1/2 inches between the flanges. The construction of the form is similar to that shown in Fig. 46 but is larger. The slots are also necessary here so that the completed section may be tied up.
About ten pounds of No. 30 B. S. gauge single silk covered wire are required to wind the secondary. The sections are wound in smooth even layers until they are six inches in outside diameter. They are then tied up and removed from the winder. The sections are separated by sheets of fiber 6 1/2 inches square on the outside, 1/2 inch thick, having a hole 4 inches square cut in the center. The sections are all connected in series and the terminals soldered to strips of copper.
After the secondary coils are all in place and connected, the fiber head is slipped on the end of the leg. Then the short lengths of the core, which had been pulled out again after squaring the core up, are slipped into place. The core is squared up again and fastened together by boring a 3/8-inch hole completely through the core at each corner. Two strips of fiber 12 x 2 1/2 x 1/2 inches are bored with corresponding holes in their ends. These strips are placed at the end of the transformer, on top of the core, and 1/4-inch bolts passed through the holes in the fiber and the core. The bolts are wrapped with micanite cloth where they pass through the core, and an insulating washer is placed under the nuts, so that the iron core laminations are not electrically connected. The nuts are tightened until the core is held firmly together.
The fiber strips also serve as insulated supports for the binding posts. The copper terminals of the secondary lead to two binding posts mounted on two fiber or hard rubber pillars 1 inch diameter and 4 inches high. The rods are arranged as explained in the section under the heading of a 1/4-K.W. transformer.
The transformer is designed for use on a 60-cycle 110-volt current. It may be used on 220 volts if the two primary coils are connected in series. When the primaries are in series, and the transformer is used on the 110-volt current, it will deliver a voltage of about 12,000 at the secondary. With either primary alone the voltage will be about 12,000, and with both in parallel about 25,000. It will then deliver a very heavy current at the secondary and draw from the line about 20 amperes in the primary. If used with a proper tuning helix, condenser and aerial, the transformer is capable of sending about 300 miles under favorable conditions.
If the transformer is to be used for long periods at a time, it is best to place it in a tight wooden box 18 inches square and 12 inches deep. The box is then filled with boiled linseed oil or amber petroleum.
A rheostat or impedance and reactance coil should be placed in series with the transformer to regulate the current and also to prevent arcing across the spark gap.
Reactance.—In Chapter I, the lag and lead of a circuit were explained in connection with tuning. This is a property of every alternating circuit and is brought to our notice again in the transformer which charges the condenser. The current developed by a transformer is a leading current, since the instantaneous values of the current do not correspond to the proportionate values of the voltage supplying the current. In order to force the current values of the charging current to correspond with the voltage it is necessary to produce a "lag." This is accomplished by means of an adjustable reactance in series with the primary of the transformer.
A reactance or inductance suitable for the 2-K.W. transformer may be made by building up a coil in the same manner as described under the heading of the J-K.W. transformer. The reactance will have to be somewhat larger on account of the heavier currents. The core is built up of sheet iron to measure 2 1/2 x 2 1/2 x 10 inches when completed. The coil is wound around a wooden form and is composed of about 100 turns of No. 8 B. S. gauge double cotton covered magnet wire. By varying the amount of core inserted in the hollow coil the energy may be adjusted as desired.
Fig. 51. Clapp-Eastham 1/4-K.W. Transformer.
Fig. 51 illustrates the 1/4-K.W. transformer manufactured by the Clapp-Eastham Company. The core is so constructed that a small metal tongue of soft iron projects from one side of the core towards the opposite side between the windings, but is separated from the opposite side by a small air gap. Several objects are accomplished by this tongue, which gives rise to magnetic leakage; the inductance of the primary is increased thereby to such an extent that the transformer is self-controlling, so that it may be connected directly to the source of alternating current supply of ordinary commercial frequencies and potential, and the current flowing in this circuit be regulated by varying the number of turns in the primary coil. As this magnetic leakage gives rise to a loose coupling effect, the primary and secondary circuits may be brought into resonance by placing a suitable capacity across the secondary terminals. This condition of resonance brings the power factor to a materially higher percentage. While the power factor of the open or closed core transformer is seldom above 50%, this type of transformer has a power factor of 80 to 90% when used with a suitable condenser.
Fig. 52. United Wireless Motor-Generator set for supplying Alternating Current to the Transformer.
Another point of considerable advantage is the almost entire freedom from arcing at the spark gap when this type of transformer is used. The spark gap is connected directly across the secondary terminals of the transformer and the condenser. The primary turns of the helix and the spark gap are connected in series. When the transformer is in operation, this condenser being across the secondary, the transformer is in resonance and the condenser is charged to such a point that it will jump the spark gap. At the instant that the spark passes, the secondary of the transformer is practically short circuited through the spark gap. As this circuit is now closed and the condenser out of circuit, the secondary circuit of the transformer is no longer in resonance and the energy immediately drops off, destroying at once the tendency for an arc to form. As soon as the spark has passed, the condenser of course comes in to play and the condition of resonance being reestablished the same process is repeated. The Clapp-Eastham Company have made application for a patent on any transformer employing this or any similar construction for use in charging a condenser.